A 245-pound linebacker who hasn't missed a game in nine seasons powers through the line, chases down the quarterback and records the 13th sack of his career. The Detroit Lions' Stephen Tulloch is so excited that he jumps as high as his battered body will take him, bending his legs to maximize the effect. When he lands, the ACL in his left knee snaps, ending his season.

The same day, a tight end who has spent much of the past year engaged in hip-strengthening exercises catches a screen pass with no defender near him. The Baltimore Ravens' Dennis Pitta stumbles as he begins to run. His body jerks to the right and he crumples to the ground. His right hip is dislocated, a rare and severe injury that threatens his career.

By now, most football observers accept that injuries are unavoidable amid such a violent game. When strong men collide at high speeds, something has to give. That understanding, however, makes it more jarring to see serious noncontact injuries result from the simplest of movements. How can this happen? Are they preventable? Or does it mean the injuries were fated to happen regardless?

I reached out to a couple of medical experts to better understand what happened in each instance. As it turns out, the injuries to Tulloch and Pitta are different on almost every level. There are some interesting theories that could explain Tulloch's mishap, but Pitta's case likely relates to a similar injury he suffered last year.

Tulloch was one of the NFL's most reliable players before Sunday, having appeared in 131 consecutive games since the Tennessee Titans drafted him in 2006. Previously, he played in 34 consecutive games at NC State and hadn't missed a game of any sort since 2003. So it was confounding to see his left leg bend awkwardly upon descent from his jump. Watching the replay, Dr. Bryan Kelly -- the chief of sports medicine and shoulder service at Hospital for Special Surgery in New York City -- was reminded of an injury that occurs often in another sport.

"In women's basketball, this is one of the most common injuries," he said. "You see players land after they jump and tear their ACLs. There is some thought that there is a mechanical difference in the way [women's] knees are formed that affects the geometry of the knee. The notch where the ACL is might be [different]."

Kelly hasn't examined Tulloch and couldn't speak specifically to the structure of his knee. Kelly did, however, point out that noncontact ACL injuries are more common in football than most people realize. According to statistics released by the NFL in January, 25 percent of torn ACLs (31 of 124) during the past three seasons occurred with no contact. (The figures don't include offseason work or practices.)